Manufacturers of electronic components use a variety of wafer processing techniques to fabricate semiconductor devices. One technique that has many applications (e.g. deposition, etching, surface cleaning, and annealing) is a dry processing technique known as "plasma-assisted" processing. In plasma-assisted processing, a substantially ionized gas, usually produced by a DC or high-frequency radio-frequency (e.g. 13.56 MHz) or microwave (e.g. 2.45 GHz) electrical discharge, generates activated metastable neutral and ionic species that chemically react to deposit thin material layers or to etch thin films on semiconductor substrates in a plasma processing reactor. Plasma processes usually are based on gas discharge in a low-pressure (0.1 mTorr to 10 Torr) process medium for improved plasma generation.
Various applications for plasma-assisted processing in semiconductor device manufacturing include reactive-ion etching (RIE) of thin films of materials such as polysilicon, metals, oxides, and polymides; dry development of photoresist layers; plasma-enhanced chemical-vapor deposition (PECVD) of dielectrics, amorphous silicon, and other materials; low-temperature chemical-vapor deposition of planarized interlevel dielectrics; and low-temperature epitaxial semiconductor growth processes. Additional applications of plasma processing techniques include physical-vapor deposition (PVD) for thin-film deposition and cryogenic plasma etch processes for high-selectivity anisotropic etching of material layers.
In plasma-assisted processing, an important process parameter is the plasma density. Plasma density is essentially defined as the number of free electrons per unit volume of plasma medium. Plasma density directly affects the concentration of activated charged and neutral species available for chemical reactions on wafer surface and semiconductor wafer processing throughput or rate in a plasma equipment. In general, a greater plasma density produces a greater process throughput due to a larger etch, deposition, or cleaning rate. Plasma density, however, can also affect the final reliability and performance of semiconductor integrated circuits fabricated based on the plasma processing techniques. For example, if process plasma density exceeds certain critical levels, the plasma medium may generate excessive concentrations of energetic species such as energetic ions and ultraviolet photons that could cause irradiation damage to the wafer surface and semiconductor devices. Therefore, it is desireable to precisely control the plasma density in a semiconductor wafer plasma processing reactor. There are several methods for plasma generation and transport to a semiconductor wafer within a plasma processing equipment. One method is to generate the plasma medium directly in the wafer processing chamber. The conventional plasma processing techniques such as RIE and PECVD usually operate based on this method by placing the semiconductor wafer between two parallel plasma-generating electrodes. Another way of producing plasma is to generate a remote plasma away from the wafer and externally to the fabrication reactor process chamber (usually by an electrodeless RF or microwave discharge). Once produced, the remote plasma stream is introduced into the process chamber and guided towards the semiconductor wafer surface.
There are several conventional ways to produce plasma at a location remote to the fabrication reactor process chamber and semiconductor wafer. One method is to excite a process gas with a fixed output electromagnetic energy source, for example, a magnetron microwave power source that generates 2.45 GHz microwave signal at a fixed output power. An advantage of such a fixed output power level microwave energy source is its simplicity and ability to produce fabrication process plasma streams cheaply relative to other adjustable microwave energy sources. The fixed power source simply operates between an off-state (no output power) and an on-state (output power ON) to provide a constant electromagnetic power to generate the plasma stream via gas discharge. With a fixed output power source, however, there is no capability to control or adjust the electromagnetic power that the process gas absorbs in the plasma production process. Without the ability to control and adjust the gas discharge electromagnetic power that the process gas medium receives it is difficult to control process plasma density and optimize various plasma process parameters such as processing rate and uniformity.
Consequently, there is a need for a device that adjustably controls the plasma-generating electromagnetic power that a fabrication process gas receives to produce a process plasma consisting of activated charged and neutral species.
The other conventional method to remotely generate plasma is to use an electromagnetic (RF or microwave) energy source with an adjustable power level. Using known technology, variable and adjustable microwave energy sources for semiconductor plasma processing comprise a variable power source and a microwave waveguide or coaxial cable inserted between a plasma-producing microwave discharge cavity and the adjustable microwave power source. The cavity usually surrounds a discharge tube (made of quartz or sapphire) through which the process gases flow. The waveguide section or the coaxial cable connects to a variable or adjustable energy source and transfers microwave power from the source to the cavity load. Although this type of system can provide adjustable discharge power, such systems are usually more expensive than the constant output magnetron power sources and take up considerable space. Moreover, because of their complexity, variable or adjustable microwave energy sources are not necessarily as reliable as are the simpler constant output power sources such as the standard stand-alone magnetron microwave power sources.
Consequently, there is a need for an inexpensive apparatus that provides plasma-generating electromagnetic energy to process gases to variably control fabrication process plasma generation, resulting in reproducible control over the device fabrication plasma process parameters.
There is yet a need for a variable or adjustable power plasma processing energy source that does not consume significant space in a semiconductor device manufacturing clean-room environment.
There is a need for a simple and reliable apparatus and method to variably provide adjustable plasma-generating energy for process plasma generation with adjustable plasma density during a semiconductor device fabrication plasma process.